An association of myasthenia gravis with Hashimoto's thyroiditis in a patient with a multinodular goitre

Introduction: The association of myasthenia gravis (MG) with other autoimmune diseases including autoimmune thyroid disease (ATD) is well recognised, although rare. The occurrence of both diseases can occur in two ways: either disease preceding the other, or concurrently. The presentation of MG in association with ATD can range from ocular to generalised disease.

Case Summary: A 26-year-old Malay female with persistent hyperthyroidism secondary to Hashimoto's thyroiditis in multinodular goitre was diagnosed with generalised MG after 2 years. She presented with right eye ptosis (ocular) and difficulty in swallowing and chewing (bulbar). The diagnosis of MG was confirmed by fatigability testing, electromyography and the presence of AChR antibodies. Her symptoms showed improvement with pyridostigmine (Mestinon) 60 mg 6-hourly. Her antithyroid drug was tapered down according to her thyroid function test. Throughout a year of follow-ups, her hyperthyroidism and fatigability symptoms improved with treatment. She was later counselled for total thyroidectomy and thymectomy.

Conclusion: Myasthenia gravis and hyperthyroidism may present with similar symptoms such as dysphagia due to neuromuscular weakness or fatigue. When the diseases occur together, one of the diagnoses may be missed. Therefore, the occurrence of new symptoms in a patient with underlying ATD should should trigger the early identification of other autoimmune diseases by primary care doctors.